Iron-based sintered alloy material and production method therefor

ABSTRACT

An iron-based sintered alloy material having, at the surface of the material, a hardened layer exhibiting a martensite phase containing a solid solution of nitrogen in a supersaturated state. The iron-based sintered alloy material may contain at least one of chromium, copper, molybdenum, manganese and nickel. A production method for the iron-based sintered alloy material includes: subjecting an iron-based sintered alloy substrate containing carbon to a nitriding treatment by heating the substrate to a nitriding temperature of at least 590° C. in an atmosphere containing ammonia, and then performing quenching by rapidly cooling the substrate.

TECHNICAL FIELD

The present invention relates to an iron-based sintered alloy material having a hardened surface and increased strength, and a production method for this iron-based sintered alloy material.

BACKGROUND ART

Surface treatments using chemical hardening methods have conventionally been used to impart metal materials with the types of material characteristics such as wear resistance and fatigue resistance required for machinery components and the like. Chemical hardening methods are methods in which a hardening component acts upon the material surface to form a hardened layer on the surface, and include a variety of treatment methods such as carburizing treatments, nitridization treatments, nitriding treatments, carbonitriding treatments, sulfonitriding treatments and boronizing treatments. Carburizing treatments represent a hardening method that has been used for a very long time, and are in widespread use, but the size of the strain generated as a result of the quenching that is conducted as a heat treatment following the carburization is problematic.

On the other hand, nitridization treatments that use precipitation strengthening by nitrides enable the treatment to be conducted with a lower heating temperature than carburizing treatments, and can therefore reduce the amount of thermal strain, but the length of the treatment time and the thinness of the hardened layer are problematic. Further, despite being hard, nitrides tend to be brittle, and therefore also suffer from problems of strength. On the other hand, nitriding treatments that employ solid solution diffusion of nitrogen do not depend on the production of nitrides, meaning the problem of brittleness can be avoided, and the degree of thermal strain is smaller than that observed in carburizing treatments. However, the drawbacks of long treatment times and shallow hardened layers still exist in nitriding treatments. For example, Patent Document 1 below describes a surface layer hardening treatment method for hardening the surface of a metal material, and discloses that by subjecting a metal material to a nitriding treatment, the Vickers hardness is increased by at least 5% down to a depth of 78 μm from the surface. A hardened layer of this depth can be obtained by treatment for 12 hours.

PRIOR ART DOCUMENTS Patent Document

-   Patent Document 1: WO 2014/104085

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

When austenite into which nitrogen has penetrated and diffused is subjected to quenching, a martensitic transformation proceeds, causing a dramatic increase in the hardness. In other words, the surface hardened layer is formed by this type of rapid cooling during heat treatment. The austenitization temperature in an Fe—N system is lower than that of an Fe—C system, and therefore the strain caused by the heat treatment in a nitriding treatment can be reduced compared to that observed in a carburizing treatment. However, as mentioned above, in conventional nitriding treatments, producing a hardened layer of adequate thickness on the surface is difficult. In order to enable machinery components and the like having excellent wear resistance and the like to be supplied efficiently and inexpensively, it is necessary to achieve surface hardening that generates little strain in the heat treatment and enables the formation of a comparatively thick hardened layer in a short period of time, thereby enabling an improvement in the material characteristics of the metal materials that constitute the machinery components and the like.

The present invention aims to address the issues described above, and has an object of providing technology that enables metal materials having improved strength provided by a hardened layer to be supplied efficiently and inexpensively, and provides high-quality products with superior precision.

Means to Solve the Problems

In order to achieve the object described above, the inventors of the present invention investigated chemical hardening methods for metal materials, and discovered that by subjecting an iron-based sintered alloy to nitroquenching, a metal material having a hardened layer formed favorably on the surface of the material could be obtained, and by utilizing this discovery, were able to develop technology capable of providing machinery components such as sprockets and various other members having excellent wear resistance and fatigue strength.

According to one aspect of the present invention, an iron-based sintered alloy material has, at the surface of the material, a hardened layer exhibiting a martensite phase containing a solid solution of nitrogen in a supersaturated state.

The iron-based sintered alloy material preferably contains 0.1 to 1.0% by mass of carbon. Moreover, the iron-based sintered alloy material may also contain at least one alloying component selected from the group consisting of chromium, copper, molybdenum, manganese and nickel. The alloying component may be any one of 0.15 to 4.5% by mass of chromium, 0.2 to 4.5% by mass of copper, 0.1 to 2.0% by mass of molybdenum, 0.1 to 3.0% by mass of manganese, and 0.2 to 4.5% by mass of nickel. The hardened layer has a depth of at least 100 μm from the surface, and therefore contributes to an improvement in bearing fatigue strength.

Further, according to another aspect of the present invention, a production method for an iron-based sintered alloy material includes molding an iron-based mixed powder containing a carbon powder into a green compact of a desired shape, obtaining an iron-based sintered alloy substrate by heating and sintering the green compact at 1,000 to 1,300° C. in a non-oxidizing environment, performing a nitriding treatment by heating the iron-based sintered alloy substrate to a nitriding temperature of at least 590° C. in an atmosphere containing ammonia, and subjecting the iron-based sintered alloy substrate that has undergone the nitriding treatment to rapid cooling and quenching.

Conducting the quenching at a quenching temperature that is lower than the nitriding temperature is effective in suppressing thermal strain. Following the quenching, performing tempering by heating the substrate at 100 to 200° C. is effective in alleviating strain and promoting the martensitic transformation of residual austenite. The carbon powder contained in the iron-based mixed powder may be 0.1 to 1.2% by mass of a graphite powder. The iron-based mixed powder preferably also contains at least one alloying component selected from the group consisting of chromium, copper, molybdenum, manganese and nickel. The iron-based mixed powder preferably contains at least one alloying component selected from the group consisting of 0.15 to 4.5% by mass of chromium, 0.2 to 4.5% by mass of copper, 0.1 to 2.0% by mass of molybdenum, 0.1 to 3.0% by mass of manganese, and 0.2 to 4.5% by mass of nickel.

Effects of the Invention

According to the present invention, an iron-based sintered alloy material having a hardened layer formed at the surface is able to provide various products such as machinery components inexpensively and with superior precision by improving characteristics such as fatigue strength and wear resistance and reducing thermal strain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a series of SEM images capturing the metal structure of cross-sections of an iron-based sintered alloy material having a hardened surface, wherein (a) represents a structure hardened by carburizing quenching, (b) represents a structure hardened by carbonitriding quenching, and (c) represents a structure hardened by nitroquenching.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Iron-based sintered alloy materials are sintered materials having an alloy composition containing iron as the main component, and are obtained by heating and sintering a green compact obtained by compression molding of a powder containing iron as the main component into a desired shape. For example, in the molding process, by conducting molding into a net shape or near net shape of the target product, the final sintered compact becomes a product formed from an iron-based sintered alloy. Sintered materials are porous materials that have pores, and the iron-based sintered alloy material of the present invention is also a porous material having pores at a porosity that corresponds with the green density during molding. If necessary, sintered materials may be subjected to processing such as sizing or coining prior to use as products, and in such cases, the surface of the sintered material is densified. Steel materials provided in forms such as molten materials, cast materials and forged materials, are widely used as the materials that constitute machinery components and structural members and the like, and iron-based sintered alloy materials having a similar alloy composition are also used in a variety of components and members. Accordingly, producing an iron-based sintered alloy material that exhibits excellent material characteristics inexpensively and efficiently is extremely useful, and by realizing an iron-based sintered alloy material that has improved material characteristics as a result of a surface hardening treatment, power transmission components and machinery equipment components and the like can be provided with high quality.

The iron-based sintered alloy material in the present invention is a surface-hardened material obtained by subjecting a sintered alloy substrate having an alloy composition similar to a steel material, namely an iron alloy composition containing carbon, to nitroquenching. By performing a nitriding treatment, nitrogen penetrates and diffuses from the alloy surface, and austenite containing nitrogen in a solid solution is produced. This austenite undergoes a martensitic transformation upon quenching, forming a martensite hardened layer containing a solid solution of nitrogen in a supersaturated state. The diffused layer of nitrogen can be formed to a depth of at least about 100 μm from the outermost surface in a comparatively short time, and the hardened layer formed from this diffused layer of nitrogen is effective in improving the bearing fatigue strength. By lengthening the nitriding treatment time, the hardened layer can be formed to even greater depth. The increase in the hardness of the surface portion provided by the hardened layer contributes to improvements in the strength and wear resistance. Because the iron-based sintered alloy substrate is a porous material, penetration and diffusion of the nitrogen proceeds not only at the outer surface of the sintered alloy substrate, but also within the pores. Accordingly, the hardened layer formed by the nitroquenching also forms on the internal surfaces of pores, namely within the depths of the sintered alloy, yielding an effect similar to the formation of a thick hardened layer. The depth of the hardened layer formed by nitroquenching of a molten material is typically about 50 μm, but in a sintered alloy material, the depth of the hardened layer can easily reach about 200 μm. With regard to quenching, achieving complete quenching is realistically difficult, and in the present invention, although nitrides can be dispersed into the martensite phase depending on the amount of nitrogen, nitride dispersion of a certain level is permissible, and does not adversely affect the functions of the hardened layer.

The austenitization temperature in an Fe—C system is about 727° C., but the austenitization temperature in an Fe—N system is at least 130° C. lower at about 590° C., and therefore the nitriding treatment can be performed at a temperature that is at least 100° C. lower than that used in a carburizing treatment. Accordingly, the quenching temperature following the nitriding treatment can be set to a lower temperature than the carburizing quenching temperature. As a result, thermal strain can be reduced dramatically compare with carburizing quenching. Moreover, because the eutectoid point in an Fe—N system (2.35% by mass N) occurs at a higher element content than the eutectoid point in an Fe—C system (0.77% by mass C), when the amount of carbon increases beyond the eutectoid point in an Fe—C system (0.77% by mass or greater), the austenitization temperature increases, whereas at the same element content in an Fe—N system, the austenitization temperature decreases as the amount of nitrogen is increased (up until 2.35% by mass). In other words, in a nitriding treatment, not only can the nitrogen undergo solid dissolution at a lower temperature, but the amount of nitrogen in the solid solution can be increased compared with the case of a carburizing treatment.

The iron-based sintered alloy material in the present invention is described below. The iron-based sintered alloy material of the present invention is a material in which a hardened layer has been formed at the surface of a sintered alloy substrate having an iron-based alloy composition containing carbon, namely an iron-based sintered alloy material having a hardened surface. Accordingly, the main portion of the material is formed from an iron-based sintered alloy containing carbon, and the surface hardened layer produced by nitroquenching exhibits a martensite phase capable of solid solution of nitrogen in a supersaturated state. The iron-based sintered alloy substrate is composed of the type of iron-based sintered alloy described below.

<Iron-Based Sintered Alloy Containing Carbon>

The iron-based sintered alloy substrate prior to hardening of the surface is composed of an iron-based sintered alloy containing carbon, and even in the iron-based sintered alloy material having a hardened surface, the portion of the material other than the hardened layer has the same alloy composition. That alloy composition is an iron alloy composition containing carbon, and includes various steel compositions such as carbon steel, low alloy steel and high alloy steel. Examples include alloy steel compositions such as chromium steel, nickel-chromium steel, nickel-chromium-molybdenum steel, chromium-molybdenum steel, nickel-molybdenum steel, manganese steel and manganese-molybdenum steel, but the composition is not limited to these compositions, and Fe—Cu—C alloys and various other iron alloy compositions containing carbon are also included within the permissible compositions. By subjecting an iron-based sintered alloy substrate having this type of composition to nitroquenching to form a hardened layer, an iron-based sintered alloy having a hardened surface can be obtained.

Production of the iron-based sintered alloy substrate prior to hardening is described below, but an iron-based sintered alloy product having this type of composition may be obtained commercially of course, with this commercial product then used as the substrate that is subjected to nitroquenching to harden the surface. The nitroquenching of the iron-based sintered alloy substrate is described below in detail.

<Production of Iron-Based Sintered Alloy Substrate>

The raw material powder used in the production of the iron-based sintered alloy substrate is a mixed powder containing a carbon powder and containing iron as the main component (an iron-based mixed powder), and depending on the desired alloy composition, may optionally contain other alloying components such as chromium (Cr), copper (Cu), molybdenum (Mo), manganese (Mn), nickel (Ni), aluminum (Al), vanadium (V), titanium (Ti) and silicon (Si). These alloying components may be included in the raw material powder in the form of an alloy powder with iron, or may be added as a separate powder. Chromium and molybdenum are components that are particularly effective in improving the material hardness and the mechanical properties. By using a raw material powder containing at least one of chromium, copper, molybdenum, manganese and nickel, iron-based sintered alloys having a composition similar to the alloy steels such as the chromium steel and nickel-molybdenum steel described above can be obtained. In those cases where one or all of these alloying components are added, the amount of each alloying component in the iron-based sintered alloy is preferably from 0.15 to 4.5% by mass of chromium, from 0.2 to 4.5% by mass of copper, from 0.1 to 2.0% by mass of molybdenum, from 0.1 to 3.0% by mass of manganese, and from 0.2 to 4.5% by mass of nickel.

If a graphite powder having an average particle size of about 1 to 40 μm is used for the carbon powder used in the preparation of the raw material powder, then the diffusion into the substrate is favorable. If a powder having an average particle size of about 1 to 300 μm, and preferably about 45 to 150 μm, is used for each of the separate powders of iron and the alloying component(s), or for the mixed powder thereof, then the powder compressibility during molding is favorable, and the production and handling of the powder are easier, which is desirable. The powders for each of the components are blended in proportions corresponding with the composition of the target iron-based sintered alloy and mixed together uniformly, and the thus obtained mixed powder is then used as the raw material powder for molding a green compact. The proportion of graphite powder in the raw material powder used for preparing an iron-based sintered alloy substrate having a carbon content of 0.1 to 1.0% by mass, taking due consideration of the air-diffused fraction, is typically from 0.1 to 1.2% by mass. Further, by also adding an appropriate amount of a powder lubricant such as a stearate salt as required, the compressibility of the raw material powder can be improved.

Molding of the raw material powder is performed by placing the raw material powder in a mold having a cavity of the desired shape, and then compressing the raw material powder using a punch or the like to mold a green compact. The molding pressure may be set appropriately in accordance with the density required of the target product, but is generally set within a range from about 250 to 800 MPa.

By subjecting the green compact obtained from the type of molding described above to heating and sintering, a sintered compact with a density of about 6.0 to 7.6 Mg/m³, namely an iron-based sintered alloy substrate, is obtained. The sintering temperature may be set to an appropriate temperature in accordance with the composition of the iron-based sintered alloy, and is generally set within a temperature range from about 1,000 to 1,300° C. If the sintering environment is an oxidizing environment, then oxidation of the sintered alloy tends to occur, and therefore the sintering is conducted in a “non-oxidizing environment for the green compact”, namely an environment that does not cause oxidation to proceed in the green compact. Specifically, the sintering is preferably conducted in a non-oxidizing atmosphere either under reduced pressure, or in an inert gas such as argon or nitrogen gas. In those cases where the green compact does not contain chromium or molybdenum, endothermic converted gas has no oxidative effect on the green compact, and therefore an endothermic converted gas may also be used as the sintering atmosphere. In other words, depending on the composition of the green compact, an endothermic converted gas may also function as a “non-oxidizing environment for the green compact”. An atmospheric gas containing hydrogen offers the advantage of reducing oxygen at the powder surface, thereby promoting the sintering process. An atmosphere having a low dew point is preferably used. By conducting this type of heated sintering, an iron-based sintered alloy substrate is obtained, and the furnace is then cooled to recover the iron-based sintered alloy substrate.

<Nitroquenching>

Nitroquenching is performed by bringing the iron-based sintered alloy substrate into contact with a nitriding gas, and requires appropriate setting of the atmospheric conditions for the iron-based sintered alloy substrate. Accordingly, around the time the iron-based sintered alloy substrate recovered from the sintering furnace is introduced into the quenching furnace, the atmospheric conditions inside the quenching furnace are set in the manner described below.

Adjustment of the atmosphere prior to conducting the nitroquenching is performed by evacuating the inside of the furnace and then re-pressurizing the furnace with nitrogen gas (nitrogen substitution) to ensure satisfactory removal of oxygen. The iron-based sintered alloy substrate is then placed inside the furnace having this adjusted atmosphere. Subsequently, vacuum evacuation is once again performed, and a low pressure of about 50 Pa is maintained for about 10 to 30 minutes, and preferably about 20 minutes. This ensures that residual gas is removed from inside the pores of the iron-based sintered alloy substrate. The furnace is then once again re-pressurized with nitrogen gas, heating is started, and the temperature inside the furnace is raised to the nitriding temperature. The nitriding temperature is set to a temperature at least as high as the austenitization temperature, namely a temperature of at least 590° C., with the nitriding typically proceeding at a temperature of about 590 to 900° C. If consideration is given to the nitriding rate and the level of thermal strain, then a temperature range of about 650 to 800° C. is preferred. The time taken to transport the iron-based sintered alloy substrate into the furnace and then perform the vacuum evacuation and temperature increase is preferably not longer than about one hour.

Once the temperature inside the furnace reaches the nitriding temperature, this temperature is maintained, and the iron-based sintered alloy substrate is held in the furnace for about 10 to 30 minutes, and preferably about 20 minutes, to enable the temperature of the entire iron-based sintered alloy substrate to reach a uniform level. Subsequently, the nitriding gas is supplied to the furnace and the nitriding treatment is started.

The nitriding treatment proceeds as a result of a nitriding gas contacting the iron-based sintered alloy substrate. A gas containing ammonia is used as the nitriding gas, with the nitriding proceeding in an atmosphere containing ammonia and nitrogen. Nitriding can also proceed in a mixed gas of ammonia and hydrogen, and therefore this type of gas may also be use. Ammonia becomes unstable when heated, and thermally decomposes into nitrogen molecules and hydrogen molecules. If steel is present, then the catalytic action of the steel causes the generation of atomic nitrogen and hydrogen only at the surface of the hot steel, with the active atomic nitrogen penetrating and diffusing into the interior of the steel. At the surface of the iron-based sintered alloy substrate that has been heated to at least the austenitization temperature (about 590° C.), active atomic nitrogen penetrates into the alloy, the nitrogen undergoes solid dissolution and diffuses (nitriding), and the surface layer portion adopts an Fe—N austenite phase. Because the nitriding proceeds in association with the above type of decomposition reaction, a mixed gas of ammonia and nitrogen gas in a ratio of 1:2 is preferably used as the nitriding gas. The nitriding progression rate is dependent on the nitrogen concentration, and the solid solubility limit for nitrogen in an Fe—N system is about 2.8% by mass N, which is larger than the solid solubility limit for carbon in an Fe—C system (2.1% by mass C). The nitriding treatment is typically conducted for about 30 to 180 minutes, and preferably about 120 to 180 minutes, and this enables the formation of a hardened layer having a depth of at least about 100 μm. The depth to which nitrogen solid solution occurs varies depending on the treatment conditions, and by lengthening the nitriding treatment time, the nitrogen is able to penetrate and diffuse deeper, resulting in an increase in the depth of the hardened layer obtained following quenching. The nitriding time is preferably set so that a hardened layer with a depth of at least about 200 μm is formed.

Rapidly cooling the Fe—N austenite phase causes a phase transformation to a martensite containing a solid solution of nitrogen in a supersaturated state (nitrogen martensite), and formation of a hardened layer having superior hardness and fatigue strength. Accordingly, by subjecting the iron-based sintered alloy substrate that has undergone nitriding to quenching, a hardened layer is formed at the surface of the substrate. The quenching temperature may be any temperature at least as high as the austenitization temperature, and may be within a range from 640 to 800° C., but in order to reduce thermal strain, the quenching temperature may be set to a temperature lower than the nitriding temperature. Accordingly, the quenching temperature is preferably set to about 640 to 720° C., and more preferably set to 660 to 700° C., with the temperature inside the furnace being reduced to the quenching temperature following the completion of the nitriding treatment. From the viewpoint of suppressing thermal strain, a rapid reduction in the temperature is preferably avoided, and the rate at which the temperature is reduced is typically set to about 0.6 to 1.0° C./minute, and preferably about 0.8° C./minute.

Once the temperature inside of the furnace has reached the quenching temperature, the iron-based sintered alloy substrate is held at the quenching temperature for about 10 to 30 minutes, and preferably about 20 minutes, to enable the temperature of the entire iron-based sintered alloy substrate to reach a uniform level. Subsequently, supply of the nitriding gas is halted, and quenching is performed by rapid cooling using a quenching liquid or gas, thereby causing hardening of the surface layer portion due to a martensitic transformation of the austenite phase. An oil or water may be used as the quenching liquid, but oil quenching using an oil of about 40 to 150° C. is preferred. An inert gas such as nitrogen or argon is preferred as the quenching gas. Cooling is conducted until the temperature of the iron-based sintered alloy substrate falls to about 50° C. or lower.

The iron-based sintered alloy material obtained following quenching has a hardened layer at the surface that exhibits a martensite phase containing a supersaturated solid solution of nitrogen. In the surface hardened layer, nitrogen exists in a solid solution, increasing the concentration. Because this iron-based sintered alloy material has been cooled from a lower quenching temperature than that employed in carburizing quenching, the thermal strain is less than that of a carburized and quenched material. When this iron-based sintered alloy material is subjected to tempering, additional strain can be removed, any residual austenite can be transformed into martensite, thereby stabilizing the structure and imparting the material with improved toughness. The tempering is preferably low-temperature tempering that can prevent embrittlement, and the tempering temperature is typically set to about 100 to 200° C., and preferably about 150 to 200° C. The tempering heating time is typically about one hour, with the tempering being able to be conducted in any one of an open (air) atmosphere, a nitrogen atmosphere and a reducing atmosphere.

The iron-based sintered alloy material obtained in the manner described above has improved hardness as a result of the hardened layer formed at the surface by the nitroquenching, and the hardened layer is formed with a depth of at least 100 μm, thereby contributing to an improvement in the bearing fatigue strength. Specifically, by subjecting an iron-based sintered alloy substrate with a hardness (Vickers hardness) of about 100 to 350 Hv to nitroquenching, the hardness at a location 0.1 mm from the surface can be increased to about 800 Hv or higher. As a result of this type of surface hardening, an iron-based sintered alloy material can be provided that exhibits improved wear resistance and reduced abrasion loss.

In the present invention, due to the difference in the austenitization temperatures, the nitriding temperature is lower than the carburizing temperature, and the quenching can also be performed at a lower temperature. Moreover, in the production method described above, by setting the quenching temperature lower than the nitriding temperature, the amount of thermal strain following quenching can be further reduced. Accordingly, the thermal strain in the obtained iron-based sintered alloy material can be halved compared with the thermal strain typically observed in carburizing quenching, meaning the dimensional precision of the product can be improved dramatically.

In this manner, by using nitroquenching, an iron-based sintered alloy material having a hardened surface can be produced with superior dimensional precision, and application to machinery components and structural members enables iron-based sintered alloy products having excellent bearing fatigue strength and wear resistance to be provided. In the case of machinery components and the like, the precision and quality required can sometimes differ depending on the field of usage, and therefore if necessary, the iron-based sintered alloy material may be subjected to appropriate processing such as sizing, coining or rolling prior to the nitroquenching. Even when this type of processing is performed, pores still remain in the densified surface, meaning nitriding can still proceed. In such cases, the material is supplied as a product composed of a porous iron-based sintered alloy material having a densified surface. Examples of alloy compositions (in which numerical values indicating composition proportions represent % by mass values) which, by implementing the present invention, enable excellent materials to be provided are described below.

(Fe—C-Based Sintered Alloys)

Iron materials contain trace amounts of unavoidable impurities caused by the production process, and carbon steel also contains trace amounts (less than 1%) of manganese and the like. Carbon steel, which is an alloy of iron and carbon containing about 0.02 to 2% of carbon, exhibits favorable toughness and is used in the production of structural components and the like for automobile components and machinery equipment and the like, but also has comparatively low hardness, and therefore surface hardening by nitroquenching is performed in order to components of improved durability to be provided. The durability of Fe—C-based sintered alloys having an alloy composition similar to carbon steel can also be improved by surface hardening using nitroquenching, and for example, by applying the technology of the present invention to sintered alloy materials having an alloy composition similar to carbon steel for machinery structures having a carbon content of 0.45% (S45C of the JIS standards) or carbon tool steel having a carbon content of 0.9 to 1.0% (SK95 of the JIS standards), machinery components and tools and the like of Fe—C-based sintered alloys having excellent durability can be provided.

(Fe—Cr—C-Based Sintered Alloys)

Chromium steel (such as SCr435, SCr440 and SCr445 of the JIS standards), stainless steel (such as SUS420 of the JIS standards), and high carbon chromium bearing steel (SUJ2 of the JIS standards) and the like contain about 0.15 to 4.5% of chromium, about 0.2 to 1.0% of carbon, and manganese as an unavoidable impurity. Alternatively, chromium-molybdenum steel (such as SCM435 and SCM 440 of the JIS standards) contain about 0.9 to 1.2% of chromium, about 0.1 to 0.2% of molybdenum, about 0.35 to 0.5% of carbon and unavoidable impurities, and because they are materials of comparatively high strength, are used as structural materials. The effectiveness of nitroquenching in Fe—Cr—C-based alloys is high, and if chromium nitrides are dispersed through the hardened layer, then the effects are enhanced. Accordingly, the durability of Fe—Cr—C-based sintered alloys can also be improved by surface hardening using nitroquenching, and by applying the technology of the present invention to sintered alloy materials having an alloy composition similar to the types of steels described above, machinery components and tools and the like of Fe—C-based sintered alloys having excellent durability can be provided.

(Fe—Cu—C-Based Sintered Alloys)

Copper steel contains about 0.2 to 4.5% of copper, about 0.4 to 1.0% of carbon and unavoidable impurities, and is used as a general structural material. By applying the technology of the present invention to a sintered alloy material having an alloy composition similar to this type of steel, Fe—Cu—C-based sintered alloys having excellent durability can be provided as general structural materials and the like.

(Fe—Ni—Mo—C-Based Sintered Alloys)

Nickel-molybdenum steel contains about 0.2 to 5.0% of nickel, about 0.1 to 2.0% of molybdenum, about 0.2 to 1.0% of carbon, and unavoidable impurities. In this composition, the nickel imparts toughness and wear resistance, and the molybdenum imparts wear resistance. The nickel and molybdenum improve the hardenability and suppress softening during tempering, and therefore by applying the technology of the present invention to a sintered alloy material having an alloy composition similar to the type of steel described above, the iron-based sintered alloy material having a formed hardened layer is able to exhibit an extremely high degree of hardness.

(Fe—Mn—Mo—C-Based Sintered Alloys)

Manganese-molybdenum steel contains about 0.1 to 3.0% of manganese, about 0.1 to 2.0% of molybdenum, and about 0.2 to 1.0% of carbon, and is a composition that has superior tensile strength.

In this composition, the manganese imparts toughness and wear resistance, and the molybdenum imparts wear resistance. The molybdenum also suppresses softening during tempering, and therefore by applying the technology of the present invention to a sintered alloy material having an alloy composition similar to the type of steel described above, the iron-based sintered alloy material having a formed hardened layer is able to exhibit an extremely high degree of hardness.

Example 1

(Sample 1)

A graphite powder was blended with an Fe—Cr—Mo—Mn alloy powder and mixed uniformly to prepare a raw material powder having an overall composition (% by mass) containing 0.5% of Cr, 0.2% of Mo, 0.2% of Mn, 0.5% of C, and the remainder as Fe. Using this raw material powder, molding and sintering were conducted in the manner described below.

A mold having a ring-shaped cavity with an outer diameter of 50 mm, an inner diameter of 30 mm and a length of 6 mm was prepared, and the raw material powder was placed in the cavity and compressed using a punch, thus forming a green compact with a green density of about 7.2 Mg/m³. This green compact was installed in a sintering furnace and heated at 1,200° C. in a mixed gas atmosphere containing 90% nitrogen and 10% hydrogen, and following sintering for 60 minutes, the temperature inside the furnace was lowered, obtaining an iron-based sintered alloy substrate of sample 1. The density was measured by the Archimedes principle, by measuring the change in weight upon immersion of the green compact in spindle oil at room temperature, and determining the density based on the obtained weight change.

(Sample 2)

A copper powder, a graphite powder and a molding lubricant were blended with an iron powder to prepare a raw material powder composed of 1.5% of Cu, 0.6% of C, and the remainder as Fe. A green product produced using this raw material powder in the same manner as sample 1 was installed in a sintering furnace and heated at 1,130° C. in a mixed gas atmosphere containing 90% nitrogen and 10% hydrogen, and following sintering for 60 minutes, the temperature inside the furnace was lowered, obtaining an iron-based sintered alloy substrate of sample 2.

(Sample 3)

With the exception of blending a nickel powder, a graphite powder and a molding lubricant with an Fe—Mo alloy powder to prepare a raw material powder composed of 1.5% of Mo, 2.0% of Ni, 0.5% of C and the remainder as Fe, and using this raw material powder, the same operations as sample 1 were repeated to obtain an iron-based sintered alloy substrate of sample 3.

(Sample 4)

With the exception of blending a Fe—Mn alloy powder, a copper powder, a graphite powder and a molding lubricant with an Fe—Mo alloy powder to prepare a raw material powder composed of 1.3% of Mn, 0.5% of Mo, 1.0% of Cu, 0.5% of C and the remainder as Fe, and using this raw material powder, the same operations as sample 1 were repeated to obtain an iron-based sintered alloy substrate of sample 4.

(Nitroquenching)

Each of the iron-based sintered alloy substrates of the above samples 1 to 4 was subjected to dimensional adjustment processing (so that the length of the sample following processing was 5.6 mm), and was then subjected to nitroquenching and tempering by performing the operations described below. In the following operations, the nitriding temperature was set to either 780° C. (samples 1 to 3) or 740° C. (sample 4), the quenching temperature was set to 700° C., and the tempering temperature was set to 180° C.

The inside of a hot-wall nitroquenching furnace was evacuated and then re-pressurized by supplying nitrogen gas, the iron-based sintered alloy substrate was placed inside the furnace, and following evacuation of the furnace for 20 minutes, nitrogen gas was supplied to re-pressurize the furnace. The inside of the furnace was then heated, and the temperature was raised to the nitriding temperature over a period of about 40 minutes. Once the nitriding temperature had been reached, the temperature was maintained for 20 minutes. Subsequently, using a mixed gas of ammonia gas and nitrogen gas (flow rate ratio=1/2) as the nitriding gas, supply of the nitriding gas was started, and the nitriding gas was brought into contact with the iron-based sintered alloy substrate to enable the nitriding treatment to proceed. The nitriding treatment was continued for 180 minutes, the temperature inside the furnace was then lowered to the quenching temperature at a cooling rate of 0.8° C./minute, and this temperature was maintained for 20 minutes. Subsequently, the supply of the nitriding gas was halted, and the iron-based sintered alloy substrate was subjected to quenching by rapid cooling using a 65° C. oil as a quenching liquid.

The iron-based sintered alloy material having a hardened surface as a result of the quenching was then subjected to tempering by heating for 60 minutes at the tempering temperature in an open atmosphere furnace, and the heating was then halted and the iron-based sintered alloy material was left to cool naturally before being recovered.

(Carburizing Quenching)

With the exceptions of using a gas carburizing agent (a coal gas containing carbon monoxide and hydrocarbons) instead of the nitriding gas, altering the heating temperature from the nitriding temperature to a carburizing temperature, and performing quenching at the carburizing temperature without any reduction in the temperature following the carburizing treatment, the same operations as those employed in the nitroquenching were repeated. The same tempering treatment was then conducted, thus obtaining iron-based sintered alloy materials produced by carburizing quenching of the iron-based sintered alloy substrates of samples 1 to 4. The carburizing temperature was set to either 850° C. (samples 1, 3 and 4) or 900° C. (sample 2).

(Measurement of Hardness)

For each of the samples 1 to 4, the hardness (HRA) of the iron-based sintered alloy material with a hardened surface was measured using a Rockwell hardness tester (ARK-F1000 manufactured by Akashi Seisakusho, Ltd.). Measurements were conducted at room temperature, under a load of 60 kgf (588 N) from a conical diamond indenter, and the average value of measurements at 5 points was obtained. Further, in the cross-section of a sample of the iron-based sintered alloy material that had been subjected to a corrosion treatment using a 5% Nital corrosive liquid, the hardness (Vickers hardness Hv) at a depth of 0.1 mm from the surface was measured (load: 0.98 N) using a Micro Hardness Testing Machine (HM-200, manufactured by Mitutoyo Corporation), and the average value of measurements at 5 points was obtained. The results are shown in Table 1.

TABLE 1 Hardness Hardness of alloy at depth Temperature material of 0.1 mm (° C.) (H_(R)A) (H_(V)) Sample 1 Nitroquenching 780 75 840 Carburizing quenching 850 73 770 Sample 2 Nitroquenching 780 74 820 Carburizing quenching 900 72 700 Sample 3 Nitroquenching 780 78 830 Carburizing quenching 850 76 755 Sample 4 Nitroquenching 740 76 810 Carburizing quenching 850 74 760

As is evident from Table 1, for each of the samples 1 to 4, it was clear that the hardness at a depth of 1 mm in the iron-based sintered alloy substrate obtained by nitroquenching was significantly improved compared with the case of carburizing quenching, indicating that surface hardening had proceeded favorably.

In the case of sample 2, when the hardness at a depth of 1.0 mm from the surface in the cross-section of the iron-based sintered alloy material was measured, the result was 700 Hv in the case of the nitroquenching and 610 Hv in the case of the carburizing quenching. These results may be regarded as indicating that the penetration and diffusion of nitrogen during the nitroquenching reached a depth of close to 1 mm.

Example 2

A mold having a cavity for molding a sprocket for a variable phase system with an over-pin diameter of 94.425 mm was prepared. An Fe—Mo—Ni alloy powder, a graphite powder and a molding lubricant were blended to prepare a mixed powder having an overall composition (% by mass) containing 0.55% of Mo, 0.55% of Ni, 0.25% of C, and the remainder as iron and unavoidable impurities, and using this mixed powder as a raw material powder, the same operations as Example 1 were conducted to produce an iron-based sintered alloy substrate having a sprocket shape. Subsequently, the teeth of the sprocket were subjected to a rolling treatment to densify the outermost surfaces of the teeth.

Using the above substrate, nitroquenching or carburizing quenching was performed in the same manner as Example 1, thus obtaining a sprocket-shaped iron-based sintered alloy material having a hardened layer formed at the surface. However, the nitriding temperature was set to 700° C. and the carburizing temperature was set to 900° C.

In addition, using the above substrate, a sprocket-shaped iron-based sintered alloy material having a hardened layer formed at the surface was obtained by subjecting the substrate to carbonitriding quenching. With the exceptions of using a gas carburizing agent containing added ammonia as the atmospheric gas for the carbonitriding instead of the gas carburizing agent used in the aforementioned carburizing quenching, and altering the heating temperature from the carburizing temperature to a carbonitriding temperature (780° C.), the carbonitriding quenching was conducted by repeating the same operations as those described above for the carburizing quenching.

SEM images capturing the metal structure of a cross-section of each of the three alloy materials obtained above are shown in FIG. 1. In FIG. 1, (a) represents the structure obtained by carburizing quenching, (b) represents the structure obtained by carbonitriding quenching, and (c) represents the structure obtained by nitroquenching. Further, when the hardness at a depth of 0.1 mm from the surface of a cross-section of each alloy material was measured, the results were 680 Hv (carburizing quenching), 680 Hv (carbonitriding quenching), and 700 Hv (nitroquenching). Moreover, when a three-ball pitching test was used to measure the bearing fatigue strength at 7.0 g/cm³ (temperature: room temperature, rotational rate: 600 min⁻¹, oil used: MTF-III, ball material: SUJ-2), the bearing fatigue strength values were 2.35 GPa (carburizing quenching), 2.35 GPa (carbonitriding quenching), and 2.40 GPa (nitroquenching).

In addition, when the strain in the sprocket shape was evaluated based on a strain measurement of the ellipticality by strain analysis, the average value for the ellipticality was 156 μm (carburizing quenching), 119 μm (carbonitriding quenching) and 60 μm (nitroquenching) respectively. It was evident that the strain upon nitroquenching was reduced to about 40% of the strain observed in the case of carburizing quenching.

The disclosure of this application is related to the subject matter disclosed in prior Japanese Application 2017-217064 filed on Nov. 10, 2017, the entire contents of which are incorporated herein by reference.

It should be noted that, besides those already mentioned above, many modifications and variations of the above embodiments may be made to the above embodiments without departing from the novel and advantageous features of the present invention. Accordingly, all such modifications and variations are intended to be included within the scope of the appended claims.

INDUSTRIAL APPLICABILITY

The present invention can provide, with superior dimensional precision, a sintered member having a hardened layer of appropriate depth at the surface of an iron-based sintered alloy by a nitriding treatment, and having excellent hardness, wear resistance and bearing fatigue strength, and therefore by applying the invention to any of various machinery components that require durability, such as sprockets, gear wheels, and shafts for rollers and motors, the resulting improvement in quality and reduction in production costs will be able to contribute to wider use of these products. 

1. An iron-based sintered alloy material comprising carbon and having, at a surface of the material, a hardened layer exhibiting a martensite phase containing a solid solution of nitrogen in a supersaturated state.
 2. The iron-based sintered alloy material according to claim 1, comprising 0.1 to 1.0% by mass of carbon.
 3. The iron-based sintered alloy material according to claim 1, further comprising at least one alloying component selected from the group consisting of chromium, copper, molybdenum, manganese and nickel.
 4. The iron-based sintered alloy material according to claim 1, further comprising at least one alloying component selected from the group consisting of 0.15 to 4.5% by mass of chromium, 0.2 to 4.5% by mass of copper, 0.1 to 2.0% by mass of molybdenum, 0.1 to 3.0% by mass of manganese, and 0.2 to 4.5% by mass of nickel.
 5. The iron-based sintered alloy material according to claim 1, wherein the hardened layer has a depth of at least 100 μm from the surface of the material.
 6. A production method for an iron-based sintered alloy material comprising: molding an iron-based mixed powder comprising a carbon powder into a green compact of a desired shape, obtaining an iron-based sintered alloy substrate by heating and sintering the green compact at 1,000 to 1,300° C. in a non-oxidizing environment, performing a nitriding treatment by heating the iron-based sintered alloy substrate to a nitriding temperature of at least 590° C. in an atmosphere comprising ammonia, and subjecting the iron-based sintered alloy substrate that has undergone the nitriding treatment to rapid cooling and quenching.
 7. The production method for an iron-based sintered alloy material according to claim 6, wherein the quenching is conducted at a quenching temperature that is lower than the nitriding temperature.
 8. The production method for an iron-based sintered alloy material according to claim 6, wherein following the quenching, tempering is performed by heating the substrate at 100 to 200° C.
 9. The production method for an iron-based sintered alloy material according to claim 6, wherein the carbon powder contained in the iron-based mixed powder is 0.1 to 1.2% by mass of a graphite powder.
 10. The production method for an iron-based sintered alloy material according to claim 6, wherein the iron-based mixed powder further comprises at least one alloying component selected from the group consisting of chromium, copper, molybdenum, manganese and nickel.
 11. The production method for an iron-based sintered alloy material according to claim 6, wherein the iron-based mixed powder further comprises at least one alloying component selected from the group consisting of 0.15 to 4.5% by mass of chromium, 0.2 to 4.5% by mass of copper, 0.1 to 2.0% by mass of molybdenum, 0.1 to 3.0% by mass of manganese, and 0.2 to 4.5% by mass of nickel. 